1. Field of the Invention
The present invention relates to a dual mode clock generator that is applicable to a direct current-direct current (DC-CD) converter of a power supply, and more particularly, to a dual mode clock generator capable of generating a suitable clock signal for an operation mode that is selected in response to selection of an active clamp forward mode and an LLC resonant half bridge mode.
2. Description of the Related Art
In general, a switching-mode power supply (SMPS) is a stabilized power supply system for converting a DC input voltage into a voltage of a square waveform using a semiconductor element, such as a power MOSFET, as a switch, to thereby obtain a DC output voltage that is controlled through a filter, wherein the stabilized power supply system has high durability and is suitably manufactured in a small and lightweight scale by controlling the flow of electric power using a switching processor of a semiconductor element.
Among these SMPS, there are several operation modes: an active clamp forward (hereinafter, referred to as ACF) mode belonging to a half bridge type; and a LLC resonant half bridge (hereinafter, referred to as LLC) mode that has attracted public attention lately, depending on the switching control system. Therefore, dedicated control devices have been developed to control each of these converters so that a plurality of dedicated ICs can drive power MOSFET in the SMPS.
For example, the ACF DC-DC converter may be considered to be one of the half bridge converters since it uses two active switches, and it has been known that power conversion capacity of the ACF DC-DC converter comes to 500 W as high as possible. This ACF DC-DC converter may be used to reduce switching loss since it operates in a soft switching mode, and to enhance its reliability by reducing voltage stress of a switch.
And, the LLC converter may be designed to be suitable for middle/large capacity power supply system since the LLC converter is basically used for a half bridge circuit, and is referred to as a power supply system that actually used to apply to 500 W or more converters and has been studied actively. Also, the LLC converter is one of the circuit systems that basically function to reduce their switching loss and conduction loss, thereby to enhance power conversion efficiency, since the LLC converter is driven in a resonance mode.
This SMPS needs a feedback control circuit to stabilize an output voltage. Here, representative examples of the feedback control circuit include a pulse width modulation (PWM) control circuit and a pulse frequency modulation (PFM) control circuit. In general, the PWM control circuit using a pulse width modulation is used in the ACF DC-DC converter, the PFM control circuit using a pulse frequency modulation is used in the LLC converter, and respective dedicated control devices are required for controlling each of the DC-DC converters.
Hereinafter, a conventional control system of the DC-DC converter will be described in detail.
Conventional SMPS is a device that converts energy by using energy storage devices such as an inductor, a transformer, a capacitor or the like. The SMPS includes a boost-up or step-up mode for converting a low input voltage to a high voltage, a step-down mode for converting a high input voltage to a low voltage, and an inverting mode for simply reversing its own polarity, etc.
Here, the DC-DC converter is a circuit that receives a DC input voltage to obtain a constant DC output voltage by suitably feeding back to the switching regulator, and it has a disadvantages that noises and ripples may be caused in the DC output voltage since the circuit transmits energy in a discontinuous packet manner every time.
These noises and ripples may be reduced by selecting suitable parts such as an inductor, a capacitor, etc. and employing suitable control methods. In this case, the conventional control methods include a pulse frequency modulation (PFM) and a pulse width modulation (PWM), as described above.
The conventional ACF DC-DC converter uses a PWM control circuit to generate a constant output voltage by generating a pulse signal whose duty is changed according to a voltage fed back from an output voltage, a reference voltage and a pulse signal waveform of an oscillator, and controlling a transistor as a switch.
In this case, the PWM control circuit may maintain a constant output voltage by increasing a pulse width when the PWM control circuit has a high load.
A clamp circuit in the DC-DC converter functions to reset a transformer, clamp a voltage of a switch, and enable zero-voltage switching (ZVS) of subsidiary and main switches to be performed.
The conventional LLC converter uses a PFM control circuit to generate a constant output voltage by turning on/off a transistor as a switch using a pulse signal of an oscillator having a fixed frequency and a constant duty, a voltage fed back from an output voltage and a reference voltage.
In this case, the PFM control circuit may maintain a constant pulse width when the PWM control circuit has a low load, and maintain a constant output voltage by continuously controlling a pulse frequency to be reduced.
The DC-DC converter has advantages that it has excellent system efficiency, and secures zero-voltage switching over the entire load range due to the high magnetizing current. And, a gate signal applied to the MOSFET switch is complementary in the LLC converter, and has a duty ratio of 50%, and the control of variable operating frequency is used to adjust an output voltage.
Accordingly, it has been known that the conventional ACF DC-DC converter operates unstably under a low load condition, and the conventional ACF DC-DC converter has been recommended to be used under a heavy load condition.
Also, it has been known that the conventional LLC converter operates unstably under a heavy load condition, and the conventional LLC converter has been recommended to be used under a low load condition.
However, each of the conventional DC-DC converters has a problem that it is impossible to apply one dedicated device to both of the ACF DC-DC converter and the LLC converter since the conventional DC-DC converters need their own dedicated control devices.
That is, it has been proposed that the dedicated control circuit that controls each of the conventional power systems operates in a PWM mode when the dedicated control circuit is subject to the heavy load, and operates in a PFM mode when the dedicated control circuit is subject to a low load. However, one problem is that the ACF DC-DC converter and the LLC converter may not be supported by the one control device.
Therefore, there has been a demand for one control circuit that can selectively support the ACF or LLC operation mode, and one clock generator that is suitable for use of the two ACF and LLC modes remains to be required for supporting both of the ACF modes and the LLC mode.